A hair coloring with acid dye has a defect such that the coloring ability is generally weaker than an oxidation hair dye. Therefore, in order to eliminate the defects, various attempts hitherto have been studied.
As a process for improving coloring ability, there has been known a method of adding benzyl alcohol, 2-benzyloxyethanol, urea, phenol, m-cresol or the like to an acidic water bath containing acid dye or the like, used in a solvent-assisted dyeing or a low-temperature dyeing in the case of wool dyeing (“Senyoku no Kiso Butsurikagaku,” authored by Mitsuo Kimura, Sennikenkyusha, p. 51–54). Among the above-mentioned components added, benzyl alcohol has been generally widely used as a penetration agent for the acid dye. However, when this method is applied to a hair coloring with acid dye, there are some defects in this process such that coloring ability and color fastness of the hair coloring with acid dye become worse than those of the oxidation hair dye.
Also, as another process for improving coloring ability, there has been proposed to use 2-benzyloxyethanol (Japanese Patent Laid-Open No. Hei 4-69323). However, there are some defects in this process such that coloring ability and color fastness are equivalent to or lower than those of the above-mentioned benzyl alcohol.
Also, as one of low-temperature dyeing processes of keratin fibers such as wool or human hair, there have been proposed a process comprising previously treating keratin fibers with an amphoteric surfactant, and thereafter dyeing the keratin fibers (Japanese Patent Laid-Open No. Hei 2-502740); the above process using a cationic surfactant in place of the amphoteric surfactant (Japanese Patent Laid-Open No. Hei 8-301738); and the like. However, there are some defects in these processes such that coloring ability is not improved, and that coloring ability especially for damaged hair is rather worsened when combined with a commercially available hair coloring with acid dye.
Incidentally, as a component for improving luster, smooth combing, feeling and the like, there has been proposed to use cationic polymers. Among them, since polyethyleneimine is alkaline, there have been proposed to use the polyethyleneimine in a hair color containing an acid dye in place of an alkalizing agent such as ammonia or monoethanolamine (Japanese Examined Patent Publication No. Hei 6-13451); a hair dye composed of a first agent comprising polyethyleneimine and a second agent comprising a melanine precursor such as tyrosine or DL-β-alanine and a metal salt (Japanese Examined Patent Publication No. Sho 61-20523); and the like.
The cationic polymer has been used not only as an agent for improving luster, smooth combing and feeling as described above, but also as a dyeing assistance for improving coloring ability. However, there has not yet been reported a case where a cationic polymer is employed in acid dye.
As the reasons therefor, there can be cited that the acidic dye has a sulfo group in the molecule, so that an ion complex is formed between the sulfo group of acid dye and cationic group when the cationic polymer exists in acid dye, whereby the acid dye cannot be efficiently penetrated into the human hair, and therefore an alkaline cationic polymer such as polyethyleneimine cannot be mixed with acid dye.
From these reasons, it has been hitherto thought that simultaneous use of the cationic polymer and acid dye would be difficult.